In the chemical analysis of grains, seeds and other agricultural commodities or solid foodstuffs such as nuts, mixed animal foods, human foods and the like, the first two stages of the analytical process are sampling and sample preparation. The initial sample is taken from "a lot" of a given foodstuff commodity. This is normally a composite made up of several probes from different parts of the lot.
The sample must be large enough to be representative of the lot. The concentration of the analyte, i.e. the chemical that is under analysis, should be the same in the initial sample as in the lot. The size of the sample taken from the lot depends on the concentration and distribution of the analyte in the foodstuff commodity.
In the case of protein, fat, moisture and some minerals which are evenly distributed, in for example, whole kernel corn at high percentage cencentrations, a half pound is enough to qualify as a representative sample. At the other extreme of low percentage concentrations are pesticides, herbicides and mycotoxins which are not evenly distributed throughout a lot of corn and can be harmful at low concentrations in the order of parts per million, i.e. ppm and parts per billion, i.e. ppb. In the case of the mycotoxin, aflatoxin, which is regulated at the 20 ppb level, 5 to 10 pounds of corn needs to be taken from a truckload to obtain a representative sample.
Sample preparation is the second stage of the analytical process and involves size reduction, such as by grinding or comminution, mixing and separating a representative analytical test sample from the initial large probe sample taken from the lot.
In the case of aflatoxin in corn, the U.S. Department of Agriculture recommends that the total large probe sample be ground using a hammer, Wiley or disk mill to pass a size 14 sieve, split using a sample splitter such as one known as a riffler, until 1 to 2 kilogram is obtained. This is reground to pass a size 20 sieve. The reground portion is mixed thoroughly in a tumble blender or planetary mixer. The analytical sample to be tested is taken from this mix using a sample splitter.
The above procedure requires a grinding mill of one type or another, a riffler and a mixer and takes about 30 minutes to obtain an analytical sample. The time required along with the various pieces of equipment employed represent a considerable investment of time and money.